President Donald Trump has lashed out over his Justice Department's appointment of a special counsel to investigate possible ties between his 2016 presidential campaign and Russia, calling it 'the single greatest witch hunt' in US history.
Deputy Attorney General Rod Rosenstein appointed Mueller after a succession of jarring developments that brought questions over Russia's alleged meddling in the election and possible collusion by the Trump campaign to a boil.
They included Trump's firing of FBI Director James Comey, reports that Trump had previously pressured Comey to end a probe of former national security adviser Michael Flynn, and Trump discussing sensitive information on Islamic State with the Russian foreign minister.
In two Twitter postings, the Republican president assailed the latest development to roil his four-month-old administration.
'With all of the illegal acts that took place in the Clinton campaign Obama Administration, there was never a special councel appointed!' Trump wrote, misspelling the word counsel as he referred to Democratic former President Barack Obama and former presidential candidate Hillary Clinton.
'This is the single greatest witch hunt of a politician in American history!' added Trump, who said in a speech on Wednesday that 'no politician in history' had been treated worse or more unfairly than he has.
Rosenstein, the No. 2 Justice Department official, named Mueller amid mounting pressure in Congress for an independent investigation beyond existing FBI and congressional probes into the Russia issue.
Mueller's appointment was not expected to affect the FBI or congressional investigations.
Senate Democratic Leader Chuck Schumer said Comey should still appear before congressional committees that have invited him to testify.
A key issue Mueller may have to tackle is whether Trump has committed obstruction of justice, an offence that could be used in any effort in Congress to impeach him and remove him from office.
Trump cited displeasure with the FBI's Russia probe as a factor in dismissing Comey. In addition, Comey wrote a memo detailing Trump's comments to him in February saying 'I hope you can let this go,' referring to the Flynn probe, a source who saw the memo said on Tuesday.